A vehicle equipped with a hydraulic booster (HB) and an anti-block brake system (ABS) in the wheel braking system or a vehicle equipped with an active suspension system has a fluid pressure source apparatus which supplies an operating pressure (high pressure). The fluid pump of the fluid pressure source apparatus is driven by an electric motor or a vehicle engine via a clutch.
A conventional fluid pressure source apparatus is disclosed in U.S. Pat. No. 5,221,125. The apparatus disclosed in this document is designed so that fluid from a reservoir is pressurized by a fluid pump driven by a motor and thereby discharged into a high pressure line. An accumulator is connected to the high pressure line to accumulate fluid pressure discharged by the pump.
A high pressure-side pressure switch and a low pressure-side pressure switch are connected to the high pressure line. Pressure detection signals generated by the pressure switches are inputted to an electronic control unit. Both the pressure switches have hysteresis characteristics. When the pressure detection signal indication from the high pressure-side pressure switch turns from high pressure to low pressure, the electronic control unit drives the fluid pump (through the electric motor). When the pressure detection signal indication from the high pressure-side pressure switch turns from low pressure to high pressure, the electronic control unit stops driving the fluid pump. Because the pressure (normal lower limit pressure) at which the indication of the pressure detection signal from high pressure to low pressure is lower than the pressure (normal upper limit pressure) at which the indication of the signal turns from low pressure to high pressure, the pressure in the accumulator is maintained between the normal lower limit pressure and the normal upper limit pressure.
The low pressure-side pressure switch is used to detect an excessive or abnormal drop of the pressure in the accumulator. If the pressure in the accumulator falls to a level (abnormal lower limit pressure) that is lower than the normal lower limit pressure, the low pressure-side pressure switch generates a pressure detection signal indicating "low pressure", whereupon the electronic control unit outputs a warning.
A current detection circuit can also be provided for detecting the value of the current fed to the electric motor. For example, it can be determined whether or not the accumulator and/or the electric motor is experiencing an abnormality based on the value of the current fed to the electric motor. In this case, the abnormality of the accumulator and/or the electric motor cannot be accurately determined if the current detection circuit is experiencing an abnormality. Therefore, it is necessary to be able to determine the existence of an abnormality in the current detection circuit.